1. Field of the Invention
The present invention relates to a photomultiplier for detecting weak light which cannot be perceived by eyes at a sensitivity reaching a quantum limit and, more particularly, to a photomultiplier for quantitatively measuring fluorescence generated from a biological microsubstance by single-photoelectron counting.
2. Related Background Art
Conventionally, various photomultipliers are available. For example, there are photomultipliers incorporating APDs (Avalanche PhotoDiode) as semiconductor devices for amplifying and detecting photoelectrons emitted from a photocathode. Such an APD, especially, a bulk-type APD is constituted by various conductive areas formed on a semiconductor substrate by diffusion or ion implantation.
Prior arts related to a photomultiplier incorporating an APD are described in detail in the following references:
U.S. Pat. No. 5,146,296, September, 1992, PA1 LASER FOCUS WORLD, pp. 125-132, November, 1993. PA1 Nucl. Instr. and Meth., vol. A310, pp. 261-266, 1991, PA1 Nucl. Instr. and Meth., vol. A315, pp. 375-384, 1992, PA1 Nucl. Instr. and Meth., vol. A330, pp. 93-99, 1993, PA1 HYBRID PHOTOMULTIPLIER TUBES, Delft Electronische Producten.
In such a photomultiplier, an avalanche multiplication gain for photoelectrons changes largely depending on the incident positions of the photoelectrons in the APD.
A prior art related to a photomultiplier incorporating an electron lens is disclosed in, e.g., U.S. Pat. No. 5,120,949.
Prior arts related to a photomultiplier incorporating a PD (PhotoDiode) in place of an APD are disclosed in detail in the following references: